1. Field of the Invention
The present invention relates generally to the field of signal processing involving signal conversion, modulation, encoding, decoding, and/or compression, and more particularly to delta modulation methods for converting and modulating an analog signal to a digital signal.
2. Description of the Prior Art
Delta modulation (DM) is an analog-to-digital signal conversion in which the analog signal is approximated with a series of segments, and each segment of the approximated signal is then compared to the original analog wave to determine the increase or decrease in relative amplitude. The decision process for establishing the state of successive bits is determined by this comparison, and only the change of information is retained, stored or sent. Specifically, conventional delta modulation describes a status of the signal in a quantized binary fashion. The status is either “1” or “0”, representing either an increase or decrease of the present status of the signal when compared to the previous status of the signal. Only an increase or decrease of the signal amplitude from the previous sample is retained. An output signal is then reconstructed (decoded) through integration (summing up) of the consecutive statuses of the signal to represent the origin of signal. The ingenuity of delta modulation technique lies in its taking advantage of the inherent data redundancy to greatly reduce the amount of data without losing much information. For this reason, delta modulation is also referred to as delta compression. Since its inception in the early 1950s, delta modulation technique has proven to be a highly advantageous signal modulation method when compared to other pulse communications systems. It is now often the de facto method used in a broader range of applications of signal processing, compression, transmitting, and data streaming. Although its most common use is for processing audio signals, delta modulation technique is not limited to such signals. In principle, delta modulation technique can be applied to any type of signal that can be digitized, including but not limited to audio signals, image signals and video signals.
Delta modulation used in communications systems usually has a differential input circuit having one input connected to a source of input signals (typically analog signals) to be encoded, and a second input to receive a feedback signal. The system also has a decoder connected to a decision circuit. The output of the decision circuit acts as the output of the delta modulation, but is additionally applied to the second input of the differential input circuit as the feedback signal to facilitate a comparison operation.
In general, the performance of a delta modulation apparatus is affected by sampling rate and choices of quantization steps. Delta modulation typically exhibits a maximum tracing slope for an input signal to be encoded. The tracing slope is limited by the product of the magnitude of the quantization step of the decision circuit and the frequency of the sampling pulses applied. For a given sampling rate (the frequency of the sampling pulses), a steeper slope of the signal (corresponding to either rapid increase or decrease of the signal amplitude) requires a greater quantization step. On the other hand, a large quantization step may produce high-level noises when applied in relatively smoother regions of the signal. To reduce this problem, adaptive delta modulation techniques, such as Continuous Variable Step Delta Modulation (CVSD), are used. These techniques use dynamically adaptive quantization steps to suit for signals having both steep slopes and smoother regions.
Despite the advancements in the delta modulation technique, further improvement is desired. One area for improvement is signal/noise ratio. For applications involving data compression, which is one of the areas where delta modulation is particularly useful, there is a constant demand for higher signal/noise ratio at a given sampling rate, or for maintaining the same signal/noise ratio at a lower sampling rate. This is because sampling rate is directly proportional to the size of data that needs to be stored or transmitted in a communications system. For narrowband implementations, for example, bandwidth is typically limited to 50 kHz or below, often in the range of 12.5 kHz–25 kHz, due to the limited transmission capacity of the system. A bandwidth of 12.5 kHz–25 kHz would mean that data rate is limited to 8 kbps–16 kbps or lower. These implementations include wireless voice communications systems such as wireless networks and walkie-talkie devices, and digital telephone systems. There is a significant need for improving performance at relatively low data rate for this type of narrowband implementations.